1. Field of the Invention
The present invention relates to the field of ultrasonic transducers, and more particularly, to the field of phased array ultrasonic transducers.
2. Background Information
Array transducers, whether they be ultrasonic transducers as in the case of ultrasonic imaging, or electromagnetic radiating horns as in the case of phase array radars, rely on wave interference for their beam forming effects. The ability to provide a focused beam on transmission and to provide a clear image on reception is dependent on each of the elements of the array having identical transduction characteristics between the electrical signals provided by the system transmitter and the wave transmitted into the medium to be explored and identical transduction functions from a wave in the medium being explored to an electrical signal provided to the signal processing system. It is only when the elements have identical characteristics that phase array combining of the signals from a plurality of elements will provide a clear image. The element characteristic which is used to compare elements is the element impulse response. That is, the element's response when a brief high amplitude electrical or wave pulse is applied to the element.
It is because of this theoretical basis for phased array processing, imaging, and coherent beam forming that phased arrays are fabricated from a plurality of elements having identical impulse responses. Since large and small objects react differently, the prior art has satisfied this requirement by using physically identical transducers in order to provide identical impulse responses.
For a number of years, ultrasound has been used as a non-invasive technique for obtaining image information about the structure of an object which is hidden from view. Ultrasound has become widely known as a medical diagnostic tool. It has also been used as a means for non-destructive testing and analysis in the technical arts.
A phased array of ultrasonic transducer elements is often designed to obtain image data. Such imaging arrays are normally linear arrays which are one element wide in the Y-direction by many elements long in the X-direction of a Cartesian coordinate system. Typically, the connections to the individual elements of a phased array ultrasonic transducer are provided by a ground conductor on a first or patient side of the piezoelectric material and individual signal electrodes on the away-from-the-patent side of the piezoelectric material. Individual signal electrode connections are normally provided by ultrasonic wire bonds to the individual signal electrodes of the individual elements. Wire bonds are a desirable means of making external connections to an ultrasonic array because they have essentially no effect on the acoustic characteristics of the transducer to which they are attached. However, a potential problem with the use of wire bonds is an inability to ensure identical placement of the wire bonds on different arrays. This can result in noise pickup and cross-talk between different signal leads which varies from one array to another, thereby complicating system design, testing and operation.
In the acoustic array transducer art, substantial effort has been expended to develop means for matching the transducer to the object to be examined and for damping any acoustic energy which emerges from the back of the transducer array and for controlling other acoustic properties of the structure in a manner to optimize ultrasonic acoustic array imager performance.
Medical ultrasonic transducer arrays normally operate at a frequency in the vicinity of 1 to 10 MHz. A linear array transducer operating at 5 MHz typically is formed from a block of piezoelectric material which is about 9 mm long by 7 mm wide by 0.4 mm thick and typically includes 64 elements. In the piezoelectric material, a frequency of 5 MHz has an acoustic wavelength of 0.8 mm which is about half the thickness of the piezoelectric body. In the human body, this acoustic frequency has a wavelength of about 0.3 mm, but varies a few percent depending on tissue type and density.
Such medical ultrasonic linear arrays are fabricated from a monolithic block of a ceramic piezoelectric material such as lead zirconate titanate (PZT) which has been made or machined to the desired shape and poled by applying an electric field of about 50,000 volts/cm across the piezoelectric in a direction perpendicular to the face of the array. Techniques for fabricating the basic PZT ceramic material are well known in the art. Once the piezoelectric block has been poled, it is coated with an electrode material such as nickel, gold or copper, gold where the first listed metal of a pair is deposited first and the second listed metal is deposited over that first metal. These metals may be deposited on the piezoelectric by processes such as evaporation, sputtering, electroless and electroplating. Plating is a preferred method of depositing the electrode material. This electrode material preferably coats all surfaces of the piezoelectric block.
An acoustic interface structure is then formed on the front (toward-the-patient) side of the block. This acoustic interface structure normally includes acoustic matching layers to transform the acoustic impedance of the piezoelectric transducer to that of its external environment. Next, the electrode material is separated into ground and signal portions by two partial depth saw cuts whose kerfs extend in the X-direction just inboard from the Y-direction edges of the block. These partial saw kerfs may preferably extend about 80% of the way through the piezoelectric block. By separating the signal and ground electrodes in this manner, the ground electrode is left accessible at the back surface of the block for connection of electrical leads, but physically spaced from the signal electrodes by the thickness of the block of piezoelectric material.
A shallow subdicing of the piezoelectric block with the dicing saw's kerfs extending in the Y-direction defines the location of each element of the final array within the piezoelectric block. This element-defining subdicing may preferably be done with a sequence of cuts of a dicing saw. This subdicing operation is done with shallow kerfs (5-25% of the depth of the piezoelectric block) in order to clearly define the elements of the transducer. This element definition subdicing separates the signal electrode portion of the metallization into individual signal electrodes. By subdicing, we mean cutting into the piezoelectric material, preferably without going all the way through it. This separates the piezoelectric into electrically separate segments, while preferably leaving it as a unitary structure.
Each signal electrode is connected to an external terminal by wire bonding. Individual element-by-element ground electrode connections are also made to a common external ground connection.
The back surface of the array and the back surface of the portion of the front acoustic control structure which extends beyond the piezoelectric block are coated as a unit with an acoustic absorber to damp any acoustic energy emerging from the back surface of the array. This is to prevent any acoustic energy which emerges from the back of the array from being reflected back into the piezoelectric material, something which would modify the acousto-electrical response of the transducer in an undesirable manner. A preferred acoustic absorber is a composite of metal particles in an attenuating soft material such as rubber, epoxy or plastic such as is disclosed in U.S. Pat. No. 4,779,244 to M. S. Horner, et al. However, other absorber may also be used. This acoustic absorber is electrically insulating and is applied as a liquid which flows easily around the wire bond wires and thus helps to prevent accidental shorting among the wire bonds during subsequent handling. Once the acoustic absorbing material has sufficiently set, the transducer is diced into its individual X-direction elements using the same or a similar dicing saw as was used to define the elements. However, this dicing is done from the front of the array. For this dicing operation, the saw blades are accurately aligned with the shallow, element-defining saw kerfs on the back of the array by means of fiducial cuts and the depth of the cut is set deep enough to ensure that X-direction-adjacent elements are physically severed from each other. It is preferred to have the saw kerfs extend beyond the back of the piezoelectric block in order to decrease acoustic cross coupling among the elements. The elements are severed by the intersection of the dicing saw's kerfs with the earlier element-defining saw kerfs. During and after this dicing operation, the individual elements are held in place by the acoustic absorber material. A thin mylar tape is then placed across the front of the array to keep debris out of the saw kerfs and to provide additional element-to-element spacing support. Following this dicing operation, the array is composed of a plurality (typically 28 to 128) individual elements which are typically about 30.lambda..sub.water long in the Y-direction by 0.5.lambda..sub.water wide in the X-direction which are separated by 0.05 mm in the X-direction.
In a manner which is well known in the phased array art, appropriate phasing of electrical signals delivered to the individual elements of this linear array can create an ultrasonic acoustic wave which forms a beam which may be steered and focused in the X-Z plane, where the Z axis is perpendicular to the X-Y plane.
During reception, the electrical signals produced by the individual acousto-electrical transducer elements of the phased array are combined in phase and amplitude in the manner which is well known in the phased array art in order to form a beam which is steered in a selected direction. Typically, the transmitted and received beams are steered in the same direction for a maximum signal-to-noise ratio and image resolution.
Focus in the Y-direction (for both transmission and reception) has normally been provided by a fixed mechanical acoustic lens with the result that the ultrasonic beam is only in proper Y-direction focus in the vicinity of a pre-established focal depth.
There has been a need for improved focus in the Y-direction. The above-identified related U.S. Pat. No. 4,890,268 discloses a solution to this Y-direction focus problem in the form of a two-dimensional array of ultrasonic transducer elements which approximates a Fresnel lens and enables electronic Y-direction focus of the ultrasonic beam as a means of overcoming this Y-direction out-of-focus problem. This array is typically fabricated from a block of material having maximum exterior dimensions of 50.lambda..sub.water by 50.lambda..sub.water by 0.5.lambda..sub.transducer, where .lambda..sub.water is the wavelength of the operating frequency in water and .lambda..sub.transducer is the wavelength of the operating frequency in the transducer material.
The two-dimensional array nature of the transducer of U.S. Pat. No. 4,890,268 complicates the use of wire bonds to provide the individual signal conductors for the various elements of the ultrasonic array. For an eight subarray structure of the type illustrated in U.S. Pat. No. 4,890,268, eight wire bonds (one for each subarray) must be made to each column of the array. For the array in U.S. Pat. No. 4,890,268, 520 signal lead bonds must be made in an area 0.6.times.0.6 inch which requires bonds at a density of 1,440 per square inch. A separate ground lead connection is also required for each column. This is beyond the capability of presently available wiring bonding systems. The close proximity of those wire bonds wires would substantially increase cross-talk, noise pickup and accidental short circuit problems. Consequently, U.S. Pat. No. 4,890,268 suggests the use of a high density interconnect technique for connection of the transducer to external terminals.
A number of high density interconnection techniques are known in the art. One of these high density interconnect structures which is also known as HDI, has been developed at General Electric Company and is the subject of a number of U.S. patents and patent applications. This General Electric high density interconnect structure has been developed in order to provide a reliable, cost effective means of interconnecting a multiplicity of electronic components such as integrated circuit chips to form a compact unitary structure for a complex (high gate count) electronic system. This HDI structure accomplishes this by doing away with individual packages for the individual chips and instead interconnecting the chips in a much denser manner.
This system overcomes the problem of low system yield which plagued previous attempts to interconnect many complex chips into an even more complex system by providing the ability to remove the high density interconnect structure in the event of an error in the high density interconnect structure itself or in the event of failure of one or more of the chips, all without any adverse effect on the good chips. Subsequently, a new high density interconnect structure may be applied to the repaired system. This repair capability is considered necessary for the interconnection of a significant number of high cost chips into a single system because chip pretesting is often incapable of establishing with certainty that chips which test "good" will, in fact, operate together in the manner intended for system. This failure to operate properly can be a result of the system operating at higher speeds that the chips are tested at, from an inability to fully specify the required chip characteristics, from an inability to measure those characteristics with sufficient accuracy or because of other causes.
This type of high density interconnect structure, methods of fabricating it and tools for fabricating it are disclosed in U.S. Pat. No. 4,783,695, entitled "Multichip Integrated Circuit Packaging Configuration and Method" by C. W. Eichelberger, et al.; U.S. Pat. No. 4,835,704, entitled "Adaptive Lithography System to Provide High Density Interconnect" by C. W. Eichelberger, et al.; U.S. Pat. No. 4,714,516, entitled "Method to Produce Via Holes in Polymer Dielectrics for Multiple Electronic Circuit Chip Packaging" by C. W. Eichelberger, et al.; U.S. Pat. No. 4,780,177, entitled "Excimer Laser Patterning of a Novel Resist" by R. J. Wojnarowski et al.; U.S. patent application Ser. No. 249,927, filed Sept. 27, 1989, entitled "Method and Apparatus for Removing Components Bonded to a Substrate" by R. J. Wojnarowski, et al.; U.S. patent application Ser. No. 310,149, filed Feb. 14, 1989, entitled "Laser Beam Scanning Method for Forming Via Holes in Polymer Materials" by C. W. Eichelberger, et al.; U.S. patent application Ser. No. 312,798, filed Feb. 21, 1989, entitled "High Density Interconnect Thermoplastic Die Attach Material and Solvent Die Attachment Processing" by R. J. Wojnarowski, et al.; U.S. patent application Ser. No. 283,095, filed Dec. 12, 1988, entitled "Simplified Method for Repair of High Density Interconnect Circuits" by C. W. Eichelberger, et al.; U.S. patent application Ser. No. 305,314, filed Feb. 3, 1989, entitled "Fabrication Process and Integrated Circuit Test Structure" by H. S. Cole, et al.; U.S. patent application Ser. No. 250,010, filed Sep. 27, 1988, entitled "High Density Interconnect With High Volumetric Efficiency" by C. W. Eichelberger, et al.; U.S. patent application Ser. No. 329,478, filed Mar. 28, 1989, entitled "Die Attachment Method for Use in High Density Interconnected Assemblies" by R. J. Wojnarowski, et al.; U.S. patent application Ser. No. 253,020, filed Oct. 4, 1988, entitled "Laser Interconnect Process" by H. S. Cole, et al.; U.S. patent application Ser. No. 230,654, filed Aug. 5, 1988, entitled "Method and Configuration for Testing Electronic Circuits and Integrated Circuit Chips Using a Removable Overlay Layer" by C. W. Eichelberger, et al.; U.S. patent application Ser. No. 233,965, filed Aug. 8, 1988, entitled "Direct Deposition of Metal Patterns for Use in Integrated Circuit Devices" by Y. S. Liu, et al.; U.S. patent application Ser. No. 237,638, filed Aug. 23, 1988, entitled "Method for Photopatterning Metallization Via UV Laser Ablation of the Activator" by Y. S. Liu, et al.; U.S. patent application Ser. No. 237,685, filed Aug. 25, 1988, entitled "Direct Writing of Refractory Metal Lines for Use in Integrated Circuit Devices" by Y. S. Liu, et al.; U.S. patent application Ser. No. 240,367, filed Aug. 30, 1988, entitled "Method and Apparatus for Packaging Integrated Circuit Chips Employing a Polymer Film Overlay Layer" by C. W. Eichelberger, et al.; U.S. patent application Ser. No. 342,153, filed Apr. 24, 1989, entitled "Method of Processing Siloxane-Polyimides for Electronic Packaging Applications" by H. S. Cole, et al.; U.S. patent application Ser. No. 289,944, filed Dec. 27, 1988, entitled "Selective Electrolytic Deposition on Conductive and Non-Conductive Substrates" by Y. S. Liu, et al.; U.S. patent application Ser. No. 312,536, filed Feb. 17, 1989, entitled "Method of Bonding a Thermoset Film To a Thermoplastic Material to Form a Bondable Laminate" by R. J. Wojnarowski; U.S. patent application Ser. No. 363,646, filed June 8, 1989, entitled "Integrated Circuit Packaging Configuration for Rapid Customized Design and Unique Test Capability" by C. W. Eichelberger, et al.; U.S. patent application Ser. No. 07/459,844, filed Jan. 2, 1990, entitled "Area-Selective Metallization Process" by H. S. Cole, et al.; U.S. patent application Ser. No. 07/457,023, filed Dec. 26, 1989, entitled "Locally Orientation Specific Routing System" by T. R. Haller, et al.; U.S. patent application Ser. No. 456,421, filed Dec. 26, 1989, entitled "Laser Ablatable Polymer Dielectrics and Methods" by H. S. Cole, et al.; U.S. patent application Ser. No. 454,546, filed Dec. 21, 1989, entitled "Hermetic High Density Interconnected Electronic System" by W. P. Kornrumpf, et al.; U.S. patent application Ser. No. 07/457,1217, filed Dec. 26, 1989, entitled "Enhanced Fluorescence Polymers and Interconnect Structures Using Them" by H. S. Cole, et al.; and U.S. patent application Ser. No. 454,545, filed Dec. 21, 1989, entitled "An Epoxy/Polyimide Copolymer Blend Dielectric and Layered Circuits Incorporating It" by C. W. Eichelberger, et al. Each of these Patents and Patent applications is incorporated herein by reference.
This high density interconnect structure has been applied to electrical circuits such as digital systems in which the electrical interconnection of elements is the significant feature of the system and the interconnection structure. Acoustic effects beyond the absence of excessive conversion of acoustic vibrations to electrical noise has not be a significant concern in this HDI structure. Consequently, what acoustic effects such as HDI structure might have on a piezoelectric array transducer are unknown. Possible effects include inducing modifications in the impulse responses of the individual elements, inducing electrical or acoustic cross-talk between adjacent elements and other effects, all of which would degrade the overall system performance of an ultrasonic acoustic array imaging system.
The introduction of a high density interconnect structure between the back of the array and acoustic damping material is undesirable from an acoustic point of view since it creates a new structure with unknown internal acoustic interactions and carries with it risks of adverse acoustic interactions which could increase noise, modify element impulse response characteristics and decrease image quality.
Acoustic and electrical interactions between the piezoelectric body of the transducer and a high density interconnect structure are unknown. Electrical or acoustic loading effects and acoustic vibrational effects may both be present with a consequent possibility of resonances which interfere with proper operation of the ultrasonic transducer array.
Accordingly, there is a need for a high density interconnection structure which is compatible with medical two-dimensional array ultrasonic transducer structures and fabrication techniques and which does not adversely affect either individual element operational characteristics or the overall operational characteristics of the phased array.